I'm not really understanding this "need to buy clothes" from those that already have a child, especially those who have at least one of each sex already. Isn't it more prudent to save your children's clothes and hand them down to the next one? Baby girls CAN wear boy clothing. I only mention shopping in my response because if I knew this baby is a girl, there have been many opportunities where I could have bought girl clothes that I like but didn't. I have clothes. I have a ton of boy clothes and even a good amount of girl clothes. I've bought some at yard sales throughout the years and while it wouldn't be enough for a full wardrobe for a little girl, she'll be alright until garage sale season starts up next year and there is no shortage of baby girl clothing to be found at low prices. The resources I "missed out from" during pregnancy will still be available after baby's birth. Even if I didn't have one shred of pink for her to wear, I've got more than enough boy clothing that doesn't say "Daddy's Little Slugger" or "100% Boy" on it. These things would do until I got the chance to get some girly gear, or at very least, a few headbands.

But then, I don't exactly buy all Gymboree or designer clothing, so maybe that's the difference. If you want all Baby Gap and Gymboree clothes, it does make sense to plan to buy during their sales events.

I also expect that if this baby is a girl, just about everyone in my life will want to gift me baby girl clothes for her, knowing that with two big brothers, I have mostly boy stuff.

We didn't find out with #4 specifically to get clothes. Dh wanted to know mostly and since I got my way with the other 3 boys, I said "okay". I was just saying it was a fun perk to get to make quilts and sew dresses and headbands when we found out we were having our first girl. And, honestly, after 3 boys; I want my baby girl in PINK, PINK, PINK! haha. My sister dresses her 3rd baby (first girl) in more neutral/almost boyish clothes and it's just not my style. We farm and have 3 boys and everything is so rough and tumble and muddy... it's nice to have a little girl be dressed up to look at (I won't even mind if she gets her pink dresses muddy).
And everyone was so thrilled we got a baby girl with #4, that I've barely had to buy any clothes for her (other than ones I really love and want her to have, tights, and then homemade stuff which I have some pretty sweet pink golden books fabric I splurged on). Some of us also have to have c-sections and between that recovery and having 3 older kids to take care of plus a newborn, I did not shop for ANYTHING post-partum for a longggggg while. So definitely no time then.
All my baby gear is gender neutral so I AM thankful for that and encourage everyone to get gender neutral gear because it's too expensive (even second hand) to have to buy for every baby!